1. Field of the Invention
The present invention relates to an optical modulator and a method for modulating the intensity of a light beam.
The present invention also relates to a unit, a transmitting station and an optical communication system comprising the optical modulator.
2. Description of the Related Art
As stated by I. P. Kaminow and T. L. Koch (“Optical fiber telecommunications IIIB”, ed. Academic Press, San Diego, 1997, pages 377-404) the earlier approach employed in digital telecommunication systems to encode the information, which is in the form of a digital electrical signal, onto to an optical carrier was direct modulation of the laser. In the typical configuration for direct modulation the electrical signal is combined with a bias current and applied to the terminals of the laser. However, when attempts are made to modulate at data bit rates beyond about 1-2 Gb/s, undesirable optical frequency variations occur, referred to as chirp. These frequency variations causes the lasing frequency to shift thereby broadening the optical spectrum of the transmitted pulses. When combined with the group velocity dispersion of standard single mode silica fibers, the sign and magnitude of the chirp for typical lasers result in an acceleration of the rate at which the pulses spread and interfere as they propagate along the fiber.
In order to overcome this problem, it has been proposed to externally modulate the intensity of a light beam emitted in continuous wave (CW) by a laser through an external optical modulator.
Typically, an optical modulator modulates at a predetermined bit rate the intensity of a light beam under the control of a digital electrical signal carrying the information to be transmitted through an optical communication system. Typically, under the control of the electrical signal, the intensity optical modulator switches a CW light beam on and off so as to obtain a “1” logic state and a “0” logic state, respectively.
Intensity optical modulators based on electro-optical effects are known in the art. These devices comprise a material the optical properties of which can be suitably changed by applying thereto an electrical field.
A Mach-Zehnder optical modulator based on an electro-optical effect is also known in the art. This modulator is provided with a interferometric configuration which converts a phase modulation into an intensity modulation. More in particular, it typically comprises a pair of waveguide arms, typically having equal length, connected between an optical waveguide splitter and an optical waveguide combiner. An input light beam is typically equally split in two light beams by the waveguide splitter, the two light beams from the splitter travel through the respective arm and they are recombined by the waveguide combiner. Electrodes are associated with the waveguide arms. By applying modulation voltage to one or both electrodes the relative phase of the two light beams can be altered through electro-optical effect. When the relative phase shift accumulated between the two light beams at the end of the two waveguide arms is π or an integer odd multiple thereof, the two light beams destructively interfere at the waveguide combiner and a “0” logic state is obtained. By contrast, when the relative phase shift between the two light beams is zero or an integer even multiple of π, the two light beams constructively interfere at the waveguide combiner and a “1” logic state is obtained.
Mach-Zehnder modulators having the two arms fabricated in lithium niobate (LiNbO3) are well known in the art (see, for example, I. P. Kaminow, T. L. Koch, “Optical fiber telecommunications IIIB”, ed. Academic Press, San Diego, 1997, pages 377-404). In these modulators the relative phase of the two light beams is altered through the well known electro-optical Pockels effect. More in particular, in these modulators the application of a modulation voltage to one or both electrodes induces—through Pockels effect—a linear variation of the refractive index of lithium niobate thereby altering the optical path of the waveguide arm(s) and, thus, the phase of the light beam(s). In order to reduce the operating voltage, the electrodes of the two waveguide arms are typically driven according to a equal push-pull configuration. That is, they are driven by modulation electric signals that induce in the two arms a variation Δ of the refractive index n which is of the same absolute value |Δn| but opposite in sign (±Δn). In this way, as long as the proper initial conditions are used, when no voltage is applied to the electrodes the relative phase shift accumulated between the two light beams at the end of the two waveguide arms is null and the “1” logic state is obtained. By contrast, when the voltage +Vπ/2 is applied to the first arm and the voltage −Vπ/2 is applied to the second arm so that the phase shift in the first arm is +π/2 while the phase shift in the second arm is −π/2, the relative phase shift accumulated between the two light beams is π and the “0” logic state is obtained. Push-pull dual arm driving condition requires a lower operating voltage than single arm driving condition. Moreover, due to the linearity of the Pockels effect, dual arm driving condition with equal push-pull configuration may give substantially zero chirp.
Richard A. Soref at al. (“Guided-Wave Intensity Modulators Using Amplitude- and Phase Perturbations”, Journal of Lightwave Technology, Vol. 6, No. 3, March 1988, pages 437-444) report a theoretical analysis of intensity modulation in coupled waveguide and Mach-Zehnder interferometers. More in particular they explore the use of combined amplitude and phase modulation induced in group IV and III-V semiconductors by electro-optical free-carrier or Franz-Keldysh effect. Moreover, they analyze three types of electro-optic guided-wave modulators: absorbing channel, Mach-Zehnder interferometer and coupled waveguide, wherein only one arm is perturbed in the last two modulators. In case of perturbation in both arms, they state that, unlike push-pull Pockels devices that give +Δβ in one arm and −Δβ in the other (wherein β=2πn/λ is the propagation coefficient), the Franz-Keldysh and charge-controlled devices give the same sign of Δβ in both arms. Hence, in the modulator with two active arms, the phase-velocity mismatch will vanish in the voltage-on-state. This implies that the phase terms will cancel and that only the loss terms will remain.
Mach-Zehnder optical modulators having the two arms fabricated in group III-V materials with multi-quantum wells are also known in the art.
For example, U.S. Pat. No. 5,524,076 discloses a Mach-Zehnder optical modulator fabricated using a group III-V material having multi-quantum wells in which the chirp parameter can be varied by adjusting the optical power splitting ratio between the two arms of the interferometer.
U.S. Pat. No. 5,778,113 discloses a Mach-Zehnder optical modulator fabricated using a group. III-V material having multi-quantum wells which can selectively generate either positive or negative frequency chirp. In fact, as stated in this document, positive frequency chirp affects long haul transmission over non-dispersion shifted fiber. In such situations, controlled levels of negative chirp provided by the optical modulator is advantageous. By contrast, some transmission networks make use of dispersion shifted or dispersion compensating optical fibers in which case negative chirp is not required and in fact it may be more appropriate to provide the data pulses with a slight positive chirp.
A modulator disclosed by U.S. Pat. No. 5,778,113 comprises an asymmetric input Y-junction waveguide coupler, an asymmetric output Y-junction waveguide coupler, a first interferometric arm defining a first optical path and a second interferometric arm defining a second optical path, wherein one of the path length is greater than the other by a distance which generates a fixed phase difference of π at the known wavelength. Electrodes are provided on the first and second arms for supplying bias and modulation voltage in push-pull mode to modulate the effective phase difference between the arms. A control electrode is provided on one of the arms for selectively applying a control electric field to selectively modulate the phase difference between the arms. With no electric field on the control electrode a negative frequency chirp is generated and with a control electric field on the arm equal to the field required to generate the fixed phase difference a positive frequency chirp is generated.
In U.S. Pat. No. 5,778,113 it is stated that one significant difference between a III-V Mach-Zehnder modulator and a LiNbO3 Mach-Zehnder modulator is that in the former absorption increases with voltage applied to the arms while absorption is not present in the latter. Moreover, a III-V Mach-Zehnder modulator shows a non-linear phase change as a function of bias voltage.
U.S. Pat. No. 5,991,471 discloses a Mach-Zehnder (MZ) optical modulator formed in a group III-V material having multi quantum wells. The modulator has first and second modulation electrode on respective first and second interferometric arm for supplying bias and modulation voltage in push-pull mode. The two modulation electrodes are of different length, the ratio of the lengths being designed to control the relative bias of the two arms, thereby providing for adjustment and optimization of frequency chirp, in a positive or negative chirp MZ modulator or in a configurable chirp MZ modulator.
The Applicant notes that the Mach-Zehnder structures disclosed by U.S. Pat. No. 5,524,076, U.S. Pat. No. 5,778,113, U.S. Pat. No. 5,991,471 require the presence of asymmetric splitter and asymmetric combiner, means for adjusting the splitting ratio of the asymmetric splitter, arms of different length and/or electrodes of different length. This may reduce the reliability of the device, increase the cost and complicate the production process.
Moreover, group III-V semiconductor materials are not compatible with silicon (Si) processing so that they cannot be integrated on a Si substrate (that is, they cannot grow on a Si substrate).
The issue of creating cost-effective silicon superchips that merge silicon photonics with advanced silicon electronics on a silicon substrate has been raised in the art. Comparing silicon substrates to group III-V platforms, silicon is available at low cost, in larger sizes (20-cm diameter wafers), with a higher degree of crystal perfection, a better native oxide, and superior mechanical and thermal properties (see Richard A. Soref, “Silicon-Based Optoelectronics”, Proceedings of the IEEE, vol. 81, no. 12, December 1993, pages 1687-1706).
In view of that, there is a need in the art of Si compatible electro-optical components so that they can be integrated on a silicon substrate.
WO 03/036367 discloses an optical modulator fabricated in SiGe and using the Franz-Keldysh effect. According to one embodiment, the modulator comprises a Mach-Zehnder structure with one or both arms made of SiGe. In this document it is stated that the absorption coefficient of the structure can be modified by means of the Franz-Keldysh effect. Because a change in the absorption coefficient will also cause a change in the phase of the light, the structure can be used to modulate light by changing the phase of the light in one or both arms. Because only a small change in absorption is needed, the composition of the waveguide region in the arms will have a SiGe content such that only a small amount of absorption will be induced by Franz-Keldysh effect.
The modulator fabricated in SiGe disclosed by this document is compatible with Si processing. The Applicant notes that the SiGe material has higher attenuation loss (compared, for example to lithium niobate) and a non-linear refractive index variation through Franz-Keldysh effect as a function of applied electric field. The Applicant further notes that due to the high attenuation loss of the SiGe material the optical modulator should be sized so as to limit the overall attenuation loss of the optical modulator. Furthermore, this document does not teach how the frequency chirp of the modulated light beam can be controlled.
As mentioned above, the frequency chirp phenomenon is a critical parameter in an optical transmission system. According to different system conditions, optical pulses having positive chirp, negative chirp or null chirp may be required to be transmitted into the optical communication system. There is, therefore, a need of an optical modulator capable of providing a modulated optical light beam with a desired frequency chirp value.